Heavy clay and rust stains need an acid based detergent for removal, OneRestore by EaCo Chem is the best that we have found, however it is expensive at $20/gallon or $100/5 gallons and you must use it straight or 1:1 applied with a pump sprayer. Note you cannot mix acid based detergents with alkaline detergents such as soaps or SH.

First pre soak any vegetation or areas nearby that are not to be cleaned includes concrete or wood floors. Apply one coat using a X-Jet without a proportioner to the red clay or rust stains. Immediately switch to rinse and begin rinsing vegetation and any areas that were misted by the detergent. After a 5 minute dwell time apply a second coat and follow by rinsing as before. After another 5 minute dwell time wash areas using high or medium pressure based on the surface being cleaned. Stubborn areas may need an addtional application after hitting with pressure.